Among the conventionally-known vehicular braking apparatus are ones provided with a simulator (i.e., operation amount simulator) for securing good operating characteristics of a brake pedal. Such braking apparatus are known, for example, from Japanese Patent No. 3489453 (hereinafter referred to as “Patent Literature 1”) and constructed in such a manner that a tread force or leg power is applied to (or caused to act on) a brake pedal, used as an operation member, so as to press a piston of a master cylinder and that a spring within the master cylinder is compressed by a fluid pressure produced within the master cylinder. With the spring within the master cylinder thus compressed, a reactive force acting on the brake pedal can be increased as a human driver presses on the brake pedal. In this way, the leg power can be adjusted in correspondence with a stroke of the brake pedal.
In the known braking apparatus, cup seals (seal members) are provided on the pistons within the master cylinder and simulator, for securing an appropriate sealing performance. These cup seals have their outer peripheral surfaces pressed against inner peripheral wall surfaces of the master cylinder and simulator. Because the outer peripheral surfaces of the cup seals are pressed against the inner peripheral wall surfaces of the master cylinder and simulator, the cup seals would produce sliding resistance when moving together with the master cylinder and simulator, and these sliding resistance acts on the brake pedal as a reactive force. Thus, in an initial operation phase or region, it becomes difficult for the pistons of the master cylinder and simulator to move, which would make it difficult to secure a good operational feeling of the brake pedal.
Further, from Japanese Patent No. 3269239 (hereinafter referred to as “Patent Literature 2”), for example, there have been known brake apparatus which are designed to enhance operability of the brake apparatus by varying a lever ratio (i.e., operation amount of the brake pedal/movement amount of a push rod) in correspondence with a stroke of the brake pedal (pedal stroke). With the brake apparatus disclosed in Patent Literature 2, the lever ratio is decreased, i.e. the movement amount of the push rod responsive to the brake pedal stroke is increased in an operation region where the braking force is small, to achieve characteristics that can facilitate stroke control. In an operation region where the braking force is great, on the other hand, the lever ratio is increased, i.e. the movement amount of the push rod responsive to the brake pedal stroke is decreased, to achieve characteristics that can facilitate leg power control. Brake pedal device of the brake apparatus disclosed in Patent Literature 2 is constructed in a manner as shown in FIG. 8 in order to decrease the lever ratio in a region where the braking force is small and increase the lever ratio in a region where the braking force is great.
FIG. 8 is a side view of the brake pedal device disclosed in Patent Literature 2. In the brake pedal device 200, a brake pedal 201 has an upper end portion 201a pivotably mounted to a vehicle body via a first pivot shaft 202, and the brake pedal 201 is connected to a push rod 205 via a connection link 203 and pivot link 204. As a leg power is applied to (acts on) a pedal 207 of the brake pedal 201 as indicated by a white arrow, the brake pedal 201 pivots clockwise about the first pivot shaft 202. The pivot link 204 is pivotably connected to the vehicle body via a second pivot shaft 206.
Behavior of the brake pedal device 200 can be schematically represented in a manner as shown in FIGS. 9A and 9B. Namely, in the schematic representation of FIGS. 9A and 9B, a leg power is applied to a pedal 207 of the brake pedal 201 as indicated by a white arrow, so that the brake pedal 201 pivots clockwise about the first pivot shaft 202. Then, a first connection arm 208 of the brake pedal 201 presses the connection link 203 as indicated by a clockwise arrow via a first connection pin 209. Also, the connection link 203 presses a first arm 204a of the pivot link 204 via a second connection pin 211, so that the pivot link 204 pivots counterclockwise about a second pivot shaft 206. Then, a second arm 204b of the pivot link 204 moves a push rod 214 via a third connection pin 213 as indicated by a leftward arrow, so that a piston 216 of a master cylinder moves as indicated by the leftward arrow.
Because the first connection arm 208 is arranged to press the connection link 203 via the first connection pin 209 as a leg power is applied to the brake pedal 207, the first connection arm 208 approaches and mechanically interfere with the first arm 204a as the pedal 207 is moved by the leg power to some degree (see FIG. 8). The first connection arm 208 thus interfering with the first arm 204a would prevent the brake pedal 207 from moving any further; namely, due to the interference, the human driver can not press on the brake pedal 207 any longer.
FIG. 10 is a graph showing relationship between a pedal operation amount and lever ratio in the conventionally-known brake pedal device, where the vertical axis represents the lever ratio while the horizontal axis represents the pedal operation amount. In a pedal operation amount region 0-s1, the lever ratio increases as the pedal operation amount increases, as indicated by a curve g1. In a pedal operation amount region s1-s2, the lever ratio decreases as the pedal operation amount increases. Further, in a pedal operation amount region s2-s3, the lever ratio increases as the pedal operation amount increases. According to the curve g1, the brake pedal device 200 is set to be used in a pedal operation amount region s4-s5. Thus, it is possible to decrease the lever ratio in an initial brake pedal operation region e1 where the braking force is small and increase the lever ratio in a following brake pedal operation region e2 where the braking force is greater.
In this case, the lever ratio has to be increased in the initial brake pedal operation region e1, in order to reduce the leg power in the initial brake pedal operation region e1. However, in the brake pedal device 200, the lever ratio is small in the initial brake pedal operation region e1, as indicated by the curve g1 of FIG. 10. Therefore, with the conventionally-known brake pedal device 200, it has been difficult to reduce the leg power in the initial brake pedal operation region. Further, because of the aforementioned interference between the connection arm 208 and the first arm 204a, the pedal 207 can be used or operated only within a range of (e1+e2) in the graph of FIG. 10; namely, the brake pedal stroke is limited to the (e1+e2) range.